120 THE WHEAT PLANT 



Golinski states that these double nuclei frequently coalesce to form a 

 single nucleus again, but I have not been able to confirm this. 



At the time of the divisions of the pollen mother-cells the tapetum 

 shows signs of disorganisation, its cells separating more or less as the 

 anther lengthens ; later, it disappears altogether, its contents breaking 

 down and furnishing nutritive material for the final development of the 

 pollen-grains. 



When the latter round themselves off and their coats become cutinised, 

 the cells of the endothecium exhibit their characteristic thickenings and 

 the exothecium or epidermis acquires its wavy-lined cuticle. 



The ovule at first consists of a hemispherical mass of meristem within 

 the carpellary cavity. Although it has the appearance of a lateral out- 

 growth on the inner posterior wall of the ovary, observation of its develop- 

 ment suggests that it is derived from the morphological apex of the floral 

 axis, its apparently lateral position being due to rapid growth of one side 

 of the axis before the closure of the carpel. 



In the earliest stages the nucellus is an almost uniform meristem, 

 usually six or seven cells across. A cell immediately beneath the epidermis 

 occupying the apex of the axial row of the mass grows into a large some- 

 what wedge-shaped archesporium. Division of the latter into two cells 

 occurs with the formation of a periclinal wall between them ; the dyad 

 divides again similarly, and a tetrad row of megaspore mother-cells is pro- 

 duced, no parietal cell being cut off the archesporium at any stage (Fig. 99). 



The outer three cells of the tetrad collapse rapidly and disintegrate, 

 the innermost remaining for a short time imbedded in the nucellus some 

 distance from the epidermis. Soon, however, the surviving mother-cell 

 enlarges greatly, and pushing apart the cells on the sides of the narrow 

 channel left by the disintegrated sister cells, ultimately comes into contact 

 with the epidermis ; later it functions as the embryo-sac. 



The germination and development of the embryo-sac up to the eight- 

 celled stage follow the usual course. It increases greatly in size and at 

 the same time absorbs some of the adjacent nucellar tissue. At the micro- 

 pylar end of the sac are two pear-shaped synergidae, each about 35-40 p, 

 long and 16-20 /x broad. The ovum, a broader cell about 50 p long and 

 32-35 /x, across, is placed by the side of these ; its nucleus is about 16 /z in 

 diameter, and at the time of fertilisation generally contains two spherical 

 nucleoli, one about 4 p, across, the other half this size. Near the ovum 

 is the smaller upper polar nucleus, the lower one being placed near the 

 three antipodals at the chalazal end of the megaspore. 



The two polar nuclei move towards each other and meet at a point 

 just behind the egg-apparatus (Fig. 100) ; here they remain in close con- 

 tact for a considerable time, and probably fuse to form the primary endo- 

 sperm nucleus just before the fertilisation of the ovum. 



